Distribution of Errors and Confidence Limits

When the concentration values obtained from repetitive measurements of rephcate samples are plotted, a normal (or Gaussian) distribution of results is often obtained. The normal curve is symmetrical about x, and the greater the s value, the wider the spread of the curve. 

When the sample size (n) is small, the values of the t-distribution are used, as shown in the following equation  

Values of the t-statistic are tabulated for commonly used confidence intervals and degrees of freedom (n— 1). 

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